G-1003 — N-Sub

The board inside N-Sub, photographed front and back — as sold by John Sands.

This Cartridge PCB uses one programmable read-only memory (PROM) from NEC named D23128C.

This is a digital memory where the setting of each bit is locked by a fuse or antifuse.

Such PROMs are used to store programs permanently. They are frequently seen in video game consoles, or such products as electronic dictionaries. For example this PROM was also find in Sinclair ZX Spectrum mainboard.

The N-Sub cartridge, as sold by John Sands — N-Sub as it was sold — John Sands

Here there are the graphic tiles relative to different game stages/screens stored on ROM: